[sdiy] Working with the ASM1 VCO.. weird MAT-02 behaviour...

[Magnus Danielson]

Hi,

Recall that as you go for very low frequencies, you get very small
currents, as you have a Volt to exponential Current converter followed
by a Current Controlled Oscillator. For the low frequencies to work, it
needs to be very clean for instance. It's not scaled to be a LFO, you
could do that, but care must be taken in such an effort.

I should dig up my ASM-1 board and measure this as I once intended to
do. I built the ASM-1, measured it with a scope to span a large range. I
then got a rubidium clock, a counter and a couple of DMMs and intended
to measure this more carefully, learned more and now have a excessive
time and frequency lab, but still have not returned to the original
project of measure the ASM-1 VCO. Also got to repair the original
rubidium clock.

Hmm, maybe I should resurrect the ASM-1 homepage I maintained, just as a
reference.

Cheers,
Magnus

On 2020-01-16 17:48, Jean-Pierre Desrochers wrote:
>
> Hi list,
>
>  
>
> I'm working on the Elby-Design ASM-1 VCO
>
> https://www.elby-designs.com/webtek/asm-1/vco/vco-asm1-cct.pdf
>
> Im want to use it as a 1v/oct LFO (.05hz to 20hz)
>
> I first mounted the circuit on my protoboard with success.
>
> at first I used a matched pair of 2N3904 for Q1A & Q1B
>
> instead of the MAT02 shown in the schematic.
>
> I could reach very low frequencies (as low as .04hz).
>
> But to do so I needed to replace R9(56k) for a 33k temporarily. Fine.
>
> This morning I replaced both 2N3904 with a genuine MAT-02
>
> and started the same frequency measurements as yesterday.
>
> I bumped into something weird..
>
>  
>
> Here are the actual measurements I first did using 2x2N3904:
>
>  
>
> mVolt at 1k TEMPCO resistor vs Sawtooth period
>
> 158.0mV => 0.050sec
>
> 176.0mV => 0.100sec
>
> 193.8mV => 0.200sec
>
> 212.0mV => 0.400sec
>
> 229.8mV => 0.800sec
>
> 247.5mV => 1.6sec
>
> 265.5mV => 3.2sec
>
> 283.2mV => 6.4sec
>
> 301.7mV => 12.8sec
>
> 319.7mV => 25.6sec
>
> (18mV steps, nice exponential response!)
>
>  
>
> Now here are the actual measurements with a MAT-02:
>
> mVolt at 1k TEMPCO resistor vs Sawtooth period
>
> 157.0mV => 0.050sec
>
> 175.0mV => 0.100sec
>
> 193.0mV => 0.200sec
>
> 211.0mV => 0.400sec
>
> 229.0mV => 0.800sec
>
> 247.0mV => 1.6sec
>
> 271.0mV => 3.2sec
>
> **** Now trying to go further increasing the TEMPCO mVolts*
>
> **** the oscillator starts backing up and re-increase its frequencies !!!*
>
> **** I could not go further lowering the frequency. *
>
>  
>
> I tried 3 different MAT-02's and even LM394's
>
> and they all behave the same.
>
> Using different matched pairs of 2N3904 worked perfectly..
>
> I suspect  MAT-02’s minimum base to emitter current reached (???)
>
>  
>
> What am-I missing here ??
>
> By the way, all the IC's I'm using here are genuine one's
>
> (TL082, MAT-02, CA3140, LM311).
>
>  
>
> JP
>
>
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